Threshold indicator



June 22, 1965 MOONEY THRESHOLD INDICATOR IN VENTOR. 80580?! aap7 FiledMarch 14. 1963 MLM United States Patent 3,191,073 THRESHOLD INDICATORLawrence K. Mooney, Alexandria, Va., assignor, by mesne assignments, tothe United States of America as represented by the Secretary of the NavyFiled Mar. 14, 1963, Ser. No. 265,296 Claims. (Cl. 307-885) The presentinvention relates to novel and improved electrical detection apparatusand more particularly to novel and improved apparatus of highsensitivity for monitoring a variable input signal for the detection ofa predetermined threshold potential.

Various types of conventional trigger circuits are often used toindicate receipt of a signal having a potential in excess of apredetermined threshold level. One circuit, which is commonly used forthis purpose, is known as the Schmitt trigger circuit which is aregenerative bistable circuit whose state depends upon the amplitude ofthe input signal. Due, however, to'a phenomenon known as hysteresiswhereby the potential at which triggering occurs depends upon whetherthe input voltage is increasing or decreasing as it crosses thethreshold potential, considerable difiiculty has been experienced in thepast in monitoring variable directcurrent voltages. Moreoverconventional threshold detection circuits devised in the past have beenincapable ofdetecting signals of either polarity in excess of thepredetermined threshold value.

It is therefore a principal object of the present invention to provide anovel and improved triggering circuit which monitors input signals whichvary at random in polarity and in potential above and below apredetermined threshold potential.

It is a further object of the present invention to provide novel andimproved highly sensitive threshold detection circuitry which minimizeshysteresis or backlash as the input signal varies about the thresholdpotential.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understoodby'reference to the following detailed description when considering inconnection with the accompanying drawing wherein:

The single figure of the drawing is a diagrammatic view of a preferredembodiment vof the present invention.

As shown in the drawing, the variable direct current input voltage iscoupled to the base of transistor T through input conductor 3. Thecollector-emitter circuit of NPN transistor T extends from the positivedirect current voltage supply line 5 through transistor T and throughresistor R to the negative direct current voltage supply line 7. Thepositive and negative voltage supply lines 5 and 7 are alsointerconnected by the voltage divider circuit that includes the seriesconnected resistor R potentiometer R potentiometer R and rheostat R Thejunction of the emitter of transistor T with resistor R is coupled at asshown to the junction of potentiometers R and R The variable arms ofpotentiometers R and R which are mechanically ganged together, arerespectively coupled to the bases of transistors T and T PotentiometersR and R are preferably of a vernier heliopot design and as will be moreapparent hereinafter thereby permit highly accurate adjustment of thedesired threshold level. The collector-emitter circuit of the NPNtransistor T in the Schmitt trigger circuit 9 extends from the positivevoltage supply line 5 through resistor R through transistor T andthrough resistor R to its junction 12 with the Schmitt trigger circuit11. The collector of transistor T is coupled to the base of the NPNtransistor T through the resistor R and resistor R The collector-emittercircuit of the NPN transistor T extends from the positive voltage supplyline 5 through the parallel arrangement of diode D and the energizingcoil 13 of relay K through transistor T and through resistors R and R tothe junction 12 with the Schmitt trigger circuit 11. Lamp 14, whichindicates the condition of energization of the emitter-collector circuitof transistor T is energized by a circuit that extends from the powersupply line 5 through contact 15 of relay K and through the lamp toground. The emitter-collector circuit of the PNP transistor T in theSchmitt trigger circuit 11 extends from its junction with the Schmitttrigger circuit 9 through resistor R through transistor T and throughresistor R to the negative voltage supply line 7. The collector oftransistor T is coupled to the base of transistor T through the resistorR and resistor R The emittercollector circuit of the PNP transistor Textends from its junction with the Schmitt trigger circuit 9 throughresistor R and resistor R through transistor T and through the parallelarrangement of diode D and the energizing coil 16 of relay K to thenegative voltage supply line .7. Lamp 17 which indicates the conditionof energization of the emitter-collector circuit of transistor T isenergized by a circuit that extends from the power supply line 7 throughcontact 18 of relay K and through the lamp to ground.

In operation the variable resistor or rheostat R is first adjusted suchthat the junction of potentiometers R3 and R at '10 isset at groundpotential. Due to the choice of magnitude of various parameters of thecircuit, the junction of Schmitt circuits 9 and 11 at 12 is also fixedat ground potential. and R are then simultaneously adjusted such thatthe cascaded Schmitt trigger circuits 9 and 11 fire at desired presetrespective positive and negative input potentials in a manner which isdescribed more fully hereinafter. The variable direct current signalwhich is to be monitored and detected at the preset threshold potentialis then'coupled to the based the emitter follower circuit of transistorTthrough input line 3. The high input impedance of the emitter followercircuit of transistor T serves to minimize loading of the circuit thatgenerates the threshold signal. Unitil the potential of the input signalon conductor 3 reaches or exceeds the preset threshold potential, theflow of current from the positive voltage supply line 5 successivelythrough resistors R R and R to junction point 12 establishes a positivepotential and a forward 'bias on the base of transistor T Thecollector-emitter circuit of transistor T therefore conducts current,the coil 13 of relay K is energized and lamp 14 is deenergized. The flowof current through resistor R develops a positive potential at theemitter of the transistor T that exceeds the positive potential at thevariable arm of potentiometer R This establishes a normal reverse biasacross the base-emitter circuit of transistor T and maintains transistorT in a normal cut-off condition. In a similar manner, before the inputsignal on conductor 3 reaches or exceeds the preset threshold potential,the flow of current from the negative voltage supply line 7 successivelythrough resistors R R and R to junction point 12 establishes a negativepotential and a forward bias on the base of transistor T Thecollector-emitter circuit of transistor T therefore conducts current,coil 16 of relay K is energized and lamp 17 is deenergized. The flow ofcurrent through resistor R develops a negative potential at the emitterof transistor T that exceeds the negative potential at the variable armof potentiometer R This establishes a normal reverse bias across thebase-emitter circuit of transistor T and maintains transistor T in anormal cut-off condition. When the input signal on line 3 is positive inpolarity and exceeds the preset threshold potential in magnitude, apositive pulse is applied to the base of the normally cut-off transistorT This provides a forward Ganged potentiometers R bias across transistorT and energizes its emitter-collector circuit. When this occurs, theresulting drop in potential at the collector of transistor T istransmitted to the base of transistor T and transistor T is cut off.This deenergizes the coil of relay K 'andenergizes the lamp 14 throughcontact of the relay. When the potential of the input signalsubsequently falls below the preset threshold potential, transistor T'deenergizes, transistor T conducts again, relay K is reenergized andlamp 14 is again deenergized. 1

Similarly, when the input signal on line 3 is negative in polarity andexceeds the preset threshold potential in magnitude, a'negative pulse isapplied to the base ofthe normally cut-off transistor T This provides aforward bias across transistor T and energizes its emitter-c01 lectorcircuit. Whenthis occurs, the resulting decrease in potential at thecollector of transistor T is transmitted to the base of transistor T andtransistor T 'is cut ofi. This deenergizes the coil of relay K andenergizes the lamp 17 through contact 18 of the relay. When the negativepotential of the input'signal falls below the preset thresholdpotential, transistor T deenergizes,

transistor T conducts again, relay K is reenergized and lamp 17 is againdeenergized.

The presence of resistors R and R in the emitter circuits of transistorsT and T respectively minimizes the hysteresis or'backlash effect in theSchmitt trigger circuits 9 and 11.

Obviously many modifications and variations of the" present inventionare possible in the light of the above It is therefore to be understoodthatwithin teachings. the scope of the appended claims the invention maybe practiced otherwise than as specifically described.

What is claimed is:

1. Threshold detection apparatus for monitoring a direct currentinputsignal of any amplitude between predetermined positive and negativepotentials, said apparatus comprising:

(a) a first Schmitt trigger circuit including a pair of NPN transistors,one of the NPN' transistors of said trigger circuit being normallyenergized and being deenergized when a predetermined positive potentialis applied to the base of'the other NPN transistor; (b) a firstindicator device;

(0) means for coupling the first indicator'device to the 1 collectorcircuit of the said one NPN transistor of the first Schmitt triggercircuit; (d) a second Schmitt trigger circuit including a pair 4 of PNPtransistors, one of the PNP transistors of said second trigger circuitbeing'normally energized and being deenergized when a predeterminednegative potential is applied to the base of the other PNP transistor;(e) a second indicator device;

(f) means for coupling the second indicator device to the collectorcircuit of the said one PNP transistor of the second Schmitt triggercircuit;

(g) a positive DC. potential source;

(h). a negative DC. potential source;

(i) a voltage divider network which is connected across said positiveand negative potential sources and which includes a pair of seriesconnected otentiometers;

(j) means connecting the variable arm of one of the potentiometers tothe base of'the said other NPN transistor;

(k) means connecting the variable arm of the other potentiometer to thebase of the said other PNP tran sistor;

(l) and means coupling the input signal to be monitored to the junctionof the series connected potentiometers. V g I 2. The apparatus set forthin. claim 1 wherein each Schmitt trigger circuit includes means forminimizing its hysteresis effect.

3. The apparatus set forth in claim 1 wherein the variable arms of thepotentiometers are mechanically;

ganged together. I

4. The apparatus set forth in claim 1 and'further comprising meansfor'maintaining the potential at the junc-- tion of the series connectedpotentiometers substantially at ground when no input signal is present.

5. The apparatus set forth in claim 1 and further comprising an emitterfollower coupled between the voltage divider network and the inputsignal.

References Cited by the Examiner V UNITED STATES PATENTS JOHN W.I-IUCKERT, Primary Examiner. ARTHUR GAUSS, Examiner.

1. THRESHOLD DETECTION APPARATUS FOR MONITORING A DIRECT CURRENT INPUTSIGNAL OF ANY AMPLITUDE BETWEEN PREDETERMINED POSITIVE AND NEGATIVEPOTENTIALS, SAID APPARATUS COMPRISING: (A) A FIRST SCHMITT TRIGGERCIRCUIT INCLUDING A PAIR OF NPN TRANSISTORS, ONE OF THE NPN TRANSISTORSOF SAID TRIGGER CIRCUIT BEING NORMALLY ENERGIZED AND BEING DEENERGIZEDWHEN A PREDETERMINED POSITIVE POTENTIAL IS APPLIED TO THE BASE OF THEOTHER NPN TRANSISTOR; (B) A FIRST INDICATOR DEVICE; (C) MEANS FORCOUPLING THE FIRST INDICATOR DEVICE TO THE COLLECTOR CIRCUIT OF THE SAIDONE NPN TRANSISTOR OF THE FIRST SCHMITT TRIGGER CIRCUIT; (D) A SECONDSCHMITT TRIGGER CIRCUIT INCLUDING A PAIR OF PNP TRANSISTORS, ONE OF THEPNP TRANSISTORS OF SAID SECOND TRIGGER CIRCUIT BEING NORMALLY ENERGIZEDAND BEING DEENERGIZED WHEN A PREDETERMINED NEGATIVE POTENTIAL IS APPLIEDTO THE BASE OF THE OTHER PNP TRANSISTOR; (E) A SECOND INDICATOR DEVICE;(F) MEANS FOR COUPLING THE SECOND INDICATOR DEVICE TO THE COLLECTORCIRCUIT OF THE SAID ONE PNP TRANSISTOR OF THE SECOND SCHMITT TRIGGERCIRCUIT; (G) A POSITIVE D.C. POTNETIAL SOURCE; (H) A NEGATIVE D.C.POTNETIAL SOURCE; (I) A VOLTAGE DEVIDER NETWORK WHICH IS CONNECTEDACROSS SAID POSITIVE AND NEGATIVE POTENTIAL SOURCES AND WHICH INCLUDES APAIR OF SERIES CONNECTED POTENTIONMETERS; (J) MEANS CONNECTING THEVARIABLE ARM OF THE OTHER POTENTIONMETERS TO THE BASE OF THE SAID OTHERNPN TRANSISTOR; (K) MEANS CONNECTING THE VARIABLE ARM OF THE OTHERPOTENTIONMETER TO THE BASE OF THE SAID OTHER PNP TRANSISTOR; (L) ANDMEANS COUPLING THE INPUT SIGNAL TO BE MONITORED TO THE JUNCTION OF THESERIES CONNECTED POTENTIOMETERS.